Beamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. The improvement compared with omnidirectional reception/transmission is known as the receive/transmit gain (or loss).
Large bandwidths available at millimeter wave frequencies for communication systems can be used to combat the exponential rise in the demand for data traffic. At millimeter wave frequencies, massive antenna arrays can be used to beamform the signal in a particular direction in order to mitigate the effects of increased path loss. Beamforming can be applied either in the digital domain or analog radio frequency (RF) domain. Digital beamforming at millimeter wave frequencies is impractical due to costly and power consuming hardware requirements, and RF beamforming alone does not capture the channel's dominant Eigen mode. Therefore, a hybrid beamforming structure is proposed for OFDM based millimeter wave systems by combining both digital and RF beamforming, where digital beamforming is applied per subcarrier before the IFFT processing at the transmitter and RF beamforming is applied per OFDM symbol in the analog domain. Similar hybrid beamforming can be applied at the receiver.
The hybrid beamforming transmission strategy involves determining a combination of digital precoder to be used at the transmitter and RF beams at each of the RF chains at both transmitter and receiver that together extract the maximum instantaneous channel capacity. The optimal codebook-based RF beam search at all transmitter and receiver RF chains has an exponential complexity in both the number of RF chains at the transmitter and that at the receiver, which is infeasible in practice.